1. Field of the Invention
The present invention relates to a method and an apparatus for producing a hermetic coated optical fiber. More particularly, the present invention relates to an improvement of a method and an apparatus for producing an optical fiber in which a bare optical fiber is coated with a hermetic coating such as a carbon coating after drawing the optical fiber.
2. Description of the Related Art
An optical fiber is fabricated by supplying an optical fiber preform made of, for example, quartz into a fiber drawing furnace from an upper opening of the furnace to melt a lower end of the preform and drawing the molten part from a lower opening of the furnace to reduce a diameter of the quartz preform to a desired diameter.
To protect a surface of the drawn bare optical fiber, a hermetic coating thin layer of a metal, an inorganic material or an organic material is provided around the surface of the bare optical fiber. Thereafter, a coating layer of a light-curable resin is applied on the coated optical fiber.
With the hermetic coating, the optical fiber is shielded from moisture in a highly humid condition such as in water so that increase of a light transmission loss due to moisture absorption can be prevented. In addition, propagation of cracks caused by water (H.sub.2 O) on the fiber surface can be suppressed, so that decrease of strength of the optical fiber due to fatique is prevented. Accordingly, the hermetic coated optical fiber attracts attention as an environment resistant fiber such as as an optical fiber for a submarine cable, and is practically used.
In general, the hermetic coating is formed on the optical fiber by a chemical vapor deposition (CVD) method in using a reactor for coating having a heating source such as a heater as means for providing a thin film on the bare optical fiber. In this connection, the CVD method for hermetic coating includes a normal pressure thermal CVD and a reduced pressure thermal CVD, but excludes a plasma CVD.
Alternatively, the hermitic coating can be formed from a carbon film generated by pyrolysis of a gas containing a hydrocarbon.
In the both methods, when the bare optical fiber is exposed to an exterior atmosphere in a space between the fiber drawing furnace and the reactor in which the hermetic coating is applied, the gas exited from the drawing furnace and the reactor is partly inhaled and recovered, whereby an air stream around the bare optical fiber is disturbed, so that the optical fiber suffers from temperature fluctuation, temperature decrease or attack by floating particles in the exterior atmosphere and cannot be coated by the hermetic coating under stable conditions. Further, turbulence of the gas stream will result in vibration of the fiber or fluctuation of the fiber diameter.
To keep the temperature of the bare optical fiber high or reduce the fluctuation of the temperature, it is proposed to use an apparatus comprising an optical fiber drawing furnace and a reactor for hermetic coating which are integrated together (of U.S. Pat. Nos. 4,702,759 and 4,790,625) or an apparatus having a shield between an optical fiber drawing furnace and a reactor (cf. Japanese Patent Kokai Publication No. 32270/1986). It is also proposed to reheat the bare optical fiber to prevent temperature decrease of the fiber before the fiber is introduced in the reactor. Further, it is disclosed in Japanese Patent Application No. 296279/1988 to prevent cooling of the bare optical fiber through heating of an inert gas to be introduced in a reactor for hermetic coating with heat of the fiber drawing furnace and the like.
The integral apparatus of the above U.S. Patents or the apparatus having the shield can advantageously prevent influence of the exterior atmosphere, keep the film forming temperature by avoiding the decrease of the surface temperature of the optical fiber and avoid inclusion of the floating particles from the exterior atmosphere into the reactor. However, since a heated purging gas from the fiber drawing furnace directly goes into the reactor and the gases in the furnace and the reactor interfere each other, it is difficult to control a temperature of the gas in the reactor or a concentration of a raw material gas for hermetic coating.
By the simple introduction of the preheated dilution gas in the reactor, it is possible to control the temperature of the gas or the concentration of the raw material in the reactor. However, if a fiber drawing rate is low or the fiber diameter is small, the bare optical fiber exiting from the fiber drawing furnace is cooled by the atmospheric air, the surface temperature of the optical fiber is lowered and a deposition temperature of the film varies, so that the control of the film properties becomes difficult.